Sunder is an upgrade in batting but not so much in bowling. He did get wickets in the end but he wasn't consistent with his line and didn't put much pressure. There is no need to replace Hardik in white ball, you can fit him and Reddy in the playing XI. Quality seam all rounders with explosive batting are saviors, get em in the squad no matter what. Having said that, Reddy needs more experience in the bowling department though.
Let me start with Sundar, he is the only spinner after Ashwin who can bowl right-arm off-spin. Being able to bat is something the selectors are looking for in that position as Ashwin is now declared all-rounder. I don't think India has any other replacement as a right-arm off-spin bowler besides Sunder.
Hardik is a prodigy but he is not fit for test format and to an extent in 50 overs matches. He is good with T20 cricket which I won't deny. Reddy is suitable for all three formats and I agree he needs experience which is the reason why he is batting down the order in the BGT series and he did take a few wickets when he was asked to bowl.
I don't think Rohit will mess up after the win under Bhumrah. He knows it is his last opportunity to create a record for himself. If he is able to win the series, I am certain he will not retire until the ICC World Test Championship. After that, he and Kohli will retire, and if he loses the BGT, we will see both of them retire from all formats of cricket.
Well known facts but both Sundar and Reddy need to up their game with the ball, IMO both of them were not threatening enough in Perth.
It wasn't a spin deck so Sundar gets the benefit of the doubt but Reddy should've done better on seam friendly pitch. There is a reason he was underbowled by Bumrah. No doubt about their batting as they are capable bat. When i am talking about Sudar and Reddy, I am only talking about their test bowling, not a white ball.
Hardik's fitness is fine in white ball be it T-20 or ODI. Injuries can happen but fitness is not the question mark at all. By any means, He's not in a race for the test team so it can't be part of any argument.